This invention relates to the separation of one or more chemical species present in an aqueous solution and more particularly to the selective separation using a series of supported liquid membranes. The separation of one or more chemical species present in an aqueous solution is very important in a variety of industrial operations. Such species include metal containing ions, neutral complexes and isomers or isotopes of the same chemical elements. Separations of these species are often associated with the extraction of valuable, toxic or radioactive metals in industrial processes and may involve the separation of toxic metals from body fluids during medical treatment. Particularly in separations in aqueous solution of species which are very similar in their physico-chemical properties, the processes may be complex and require repeated stages with replenished extraction liquids to obtain the desired separation. A typical separation process involving multistages or steps is liquid-liquid extraction where a fresh second liquid is utilized repeatedly to achieve a degree of selective separation.
Separation processes have also utilized certain membranes for separating and for concentrating ionic species. In U.S. Pat. No. 3,454,490, cation and anion selective membranes are disclosed in a process where the chemical potential of ionic species on opposite sides of a membrane provides the driving force for the separation. In other processes such as that disclosed in U.S. Pat. No. 4,292,181, an internal phase in a liquid membrane as an emulsion is used to remove a component from an aqueous solution.
Some separation processes have utilized a carrier-containing supported liquid membrane in a single stage operation, where the membrane provides isolation of the aqueous solution from a second aqueous solution. The membrane usually is characterized by a mobile carrier or extractant, such as a long chain alkylphosphoric acid, a phosphoric acid ester or a tertiary amine, dissolved in an organic diluent absorbed on a thin, microporous, polymeric film. The resulting membrane provides a medium which extracts the species from the adjacent solution by forming a membrane soluble complex with the carrier. After extraction into the membrane, the soluble complex diffuses through the membrane and is back extracted into another aqueous solution (stripping solution) of different composition. With two or more aqueous species, the permeability coefficients of each species will determine the relative amounts of the different species which after a given time are extracted from the solution and transferred to the other aqueous solution, present on the opposite side of the membrane. As indicated, this process is limited to single stage separations since it is necessary to change the chemical composition of the stripping solution for continuous operation of the separation process. Therefore, multistage separation processes using carrier-containing supported liquid membranes have not been considered feasible because the composition and/or concentrations of the solutions on opposite sides of the membrane are required to be different.
In fact, chemical species permeate carrier-containing supported liquid membranes, even against their concentration gradient, at the expense of an opposite concentration gradient. This is maintained practically constant during the process either by the use of different and large concentrations (with respect to the species which have to be separated by the membrane) of the same aqueous electrolyte, which is solubilized in the two solutions present on the two opposite sides of the membrane, or by the use of two completely different electrolytes. Moreover, while the first aqueous electrolytic solution, initially containing the species to be separated, must be such to promote extraction into the membrane, the opposite aqueous electrolytic solutions must be such to promote the stripping from the membrane of the species which have initially entered into the membrane. It follows that if another membrane of the same nature, i.e., containing the same carrier at the same concentration, is again contacted with the aqueous electrolytic solution which acted as stripping solution of the species to be separated from the first membrane, no further extraction of these species into this second membrane can take place. Consequently the process stops and the separation of the membrane permeating species can only be operated as a single stage process.
Accordingly, one object of the invention is a multistage process for separating one or more chemical species present in an aqueous solution. Another object of the invention is a multistage process utilizing simple aqueous solutions for the separation. A further object of the invention is a continuous multistage separation process. These and other objects will become apparent from the following description.